An unusually incurable disorder in which the infected derp uncontrollably and prevents them from being a functional member of society or something of that nature.
Jeeze, that person can't do anything right they must have derpatitis! -Dr. Adkins.
Having an excessive case of derpies or herp derp, where the infected replaces common words with herp or derp.
Derpatitis is commonly heard during talking or singing "ahh, herp herp herp herp herp ahh derp" or more commonly sung as "Ah, ba ba ba ba Barbara Ann"